Saccade initiation and the reliability of motor signals involved in the generation of saccadic eye movements.

We examined the trial-by-trial relationship between the metrics of saccadic eye movements and the activity of individual putative premotor neurons in the paramedian pontine reticular formation (PPRF) of rhesus monkeys. The region of the pons containing these excitatory burst neurons (EBNs) extends for several millimetres. Motoneurons innervating extraocular muscles integrate the output of hundreds or even thousands of these neurons. Accordingly, there was no reason to expect that relatively small variations in the activity of a single pontine neuron would be related to variations in saccade amplitude or speed observed during repetitive eye movements to the same target. Nonetheless, we observed consistent relationships between variations in the number of spikes in the burst of pontine neurons and the amplitude of the saccade. Trial-to-trial variations in the instantaneous spike frequency during a burst are associated with variations in the velocity profile of the movement. Based on these data, we conclude that the activity of pontine burst neurons is not statistically independent and that simultaneous recordings from multiple cells will reveal a high degree of correlated activity.

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